Procollagen and fibronectin are major products of confluent fibroblasts in culture and both are released from the cells. Procollagen is secreted by known pathways, while the mechanism of fibronectin release is controversial. We find that the secretion of both these proteins can be reduced to 20% by low concentrations (0.1-1 muM) of ionophores that have affinity for monovalent cations. In contrast, little effect upon secretion was found for similar concentrations of an ionophore that binds divalent cations. Electron microscopy showed that the inhibition of secretion is accompanied by accumulation of membranous vacuoles. We believe that the ionophores impede secretion by acting on the secretory structures rather than on the proteins themselves. Biochemical studies supported this interpretation because no changes were detected in hydroxylation or glycosylation of procollagen or glycosylation of fibronectin, nor were significant changes in cellular amino acid incorporation observed. Pulse-chase studies indicated that the rates of secretion were impaired by the ionophore without enhancing intracellular degradation. The decreased secretory rates accounted for the lower levels of procollagen and fibronectin in the culture medium; no evidence for increased catabolism of the secreted proteins was found. Secretion could be readily restored by removing the ionophore from the culture medium. The results indicate that procollagen and fibronectin may be simultaneously secreted, possibly utilizing a common pathway for secretion; the ionophores effectively interfere with cellular secretory pathways without impairing protein synthesis or protein glycosylation or altering protein catabolism.